The Stirling cycle engine was originally conceived during the early portion of the nineteenth century by Robert Stirling. During the middle of the nineteenth century, commercial applications of this hot gas engine were devised to provide rotary power to mills. The Stirling engine was ignored thereafter until the middle of the twentieth century because of the success and popularity of the internal combustion engine. Stirling cycle machines, including engines and refrigerators, are described in detail in Walker, Stirling Engines, Oxford University Press (1980), incorporated herein by reference.
The principle underlying the Stirling cycle engine is the mechanical realization of the Stirling thermodynamic cycle: 1) isovolumetric heating of a gas within a cylinder, 2) isothermal expansion of the gas (during which work is performed by driving a piston), 3) isovolumetric cooling and 4) isothermal compression. Additional background regarding aspects of Stirling cycle machines and improvements thereto are discussed in Hargreaves, The Phillips Stirling Engine (Elsevier, Amsterdam, 1991), incorporated herein by reference.
The high theoretical efficiency of the Stirling engine has attracted considerable interest in recent years. The Stirling engine adds the additional advantages of easy control of combustion emissions, potential use of safer, cheaper, and more readily available fuels and quiet running operation, all of which combine to make the Stirling engine a highly desirable alternative to the internal combustion engine for many applications.
Despite these advantages, development of the Stirling engine has proceeded at a much slower rate than might otherwise be expected. Some of the more acute problems include the need to seal the working gas at a high pressure within the working space, the requirement for transferring heat at high temperature from the heat source to the working gas through the heater head, and a simple, reliable and inexpensive means for modulating the power as the load changes.
One design, which is well suited to a variety of applications, is the free-piston Stirling engine. The free-piston Stirling engine uses a displacer that is mechanically independent of the power output member. Its motion and phasing relative to the power output member is accomplished by the state of a balanced dynamic system of springs and masses, rather than a mechanical linkage.
Stirling engines have been proposed for use in a wide range of applications. Examples include automotive applications, refrigeration systems and applications in outer space. The need to power portable electronics equipment, communications gear, medical devices and other equipment in remote field service presents yet another opportunity, as these applications require power sources that provide both high power and energy density, while also requiring minimal size and weight, low emissions and cost.
To date, batteries have been the principal means for supplying portable sources of power. However, the time required for recharging batteries has proven inconvenient for continuous use applications. Moreover, portable batteries are generally limited to power production in the range of several milliwatts to a few watts and thus cannot address the need for significant levels of mobile, lightweight power production.
Small generators powered by internal combustion engines, whether gasoline- or diesel-fueled have also been used. However, the noise and emission characteristics of such generators have made them wholly unsuitable for a wide range of mobile power systems and unsafe for indoor use. While conventional heat engines powered by high energy density liquid fuels offer advantages with respect to size, thermodynamic scaling and cost considerations have tended to favor their use in larger power plants.
In order to execute the Stirling cycle, either for the purpose of making power as in an engine embodiment or for the purpose of refrigeration as in a cooler embodiment, the machine must be provided with both an external heat source and an external heat sink. Heat transfer between the external pressure vessel wall of the machine and the working fluid is typically accomplished through the use of internal heat exchangers. Maximum efficiency is obtained when as much heat as possible is transferred to the working fluid rather than to engine components or other heat absorbers.
Heat transfer to the working fluid is affected by three heat exchanger characteristics: 1) the surface area of the heat exchanger that is in contact with the heat source/sink and the working fluid, 2) the heat transfer coefficient between the working fluid and the surface, and 3) the temperature differential between the heat exchanger surface and the working fluid. Improved heat transfer can be effected by increasing any or all of these three parameters.
The desire for high thermal efficiencies in Stirling engines dictates high regenerator effectiveness and as a result, the fluid exiting the regenerator and entering the hot end heat exchanger, henceforth referred to as the “heater”, is at or near the temperature of the heater walls. Similarly, the temperature of the working fluid exiting the regenerator and entering the cold end heat exchanger, henceforth referred to as the “cooler”, is at or near the temperature of the cooler walls. Further, since engine pressure variations are typically low, particularly in free-piston Stirling machine embodiments, end state expanded or compressed fluid temperatures tend towards the heater and cooler wall temperatures, respectively. Additionally, working fluid temperatures within conventional heater or cooler heat exchangers vary spatially with the maximum temperature differential between the working fluid and the heat exchanger walls at the respective heat exchanger inlets, and decreasing along the lengths of the heat exchangers until reaching a minimum at the heat exchanger exit where, if the heat exchangers are reasonably well designed, the working fluid has reached very nearly the heat exchanger wall temperature. As a result, the effective temperature differentials between the heater and cooler heat exchangers and working fluid in well designed Stirling cycle machines are, by design small.
Conventional heat exchanger designs for Stirling cycle machines typically employ slots or holes inside of thick walls, tubes, or alternatively, extended surfaces within channels such as fins of various types. Traditional heat exchanger designs which are used within Stirling cycle machines operate with a low temperature differential between the heat source and the working fluid as discussed above. In order to compensate for the low temperature differentials between the heat source or sink and the working fluid, traditional heat exchanger applications in connection with Stirling cycle machines have suffered from other tradeoffs.
For example, in some cases heat exchanger structures must be larger than desirable in order to provide the necessary increased surface area for effective heat transfer. This, in turn results in larger engine sizes, less space for other engine components, or both. Additionally, in some cases solutions designed to achieve the necessary heat transfer have required the use of expensive, and sometimes exotic, materials as well as expensive, time-consuming and sometimes less than reliable manufacturing processes and designs.
Other drawbacks also exist with prior art heat exchanger designs. For example, the requirement for metal-to-metal contact between the pressure vessel and the heat exchanger walls to achieve minimal thermal resistance results in designs that are difficult to fabricate and thus expensive.
In lieu of providing a large surface area to achieve the required heat transfer, the heat exchanger may be designed to generate high heat transfer coefficients, albeit at the expense of somewhat higher pressure drops in the heat exchanger. However, significant manufacturing, assembly and cost benefits accrue from eliminating the need for extended surface heat exchangers.